Experimental Search for Quantum Gravity 2007

During the last few years, various possibilities to experimentally test quantum gravity within the near future have been proposed, based on effective models that incorporate features like deformations of special relativity, the existence of a minimal length, decoherence due to space-time foaminess, or leftover imprints of quantum effects from the early universe. With this workshop, we want to bring together people from different areas in order to assess these possibilities and encourage discussions.

The scope of this meeting is focused on possible experimental tests and their relation to quantum gravity. Questions that will be addressed are: which assumptions go into the predictions, what progress has been made during the last few years, and what insights could be gained from experimental feedback for the underlying theory.

The program will includes the following topics:

Predictions for existent and planned earth based experiments (accelerators, high-precision measurements), possibly in scenarios with a lowered Planck scale.

Cosmological measurements: leftover signatures from the universe's early quantum phase (e.g. in the cosmic microwave/neutrino/graviton background), large and small scale structure, etc.

I discuss the status of Quantum Gravity Phenomenology, focusing separately on the 3 key areas: ability to discover, ability to constrain, and ability to falsify. And I stress the importance of adopting carefully taylored test theories as a remedy to difficulties encountered when comparing experimental evidence to theory evidence.

Brandenberger, Robert

Probing string theory through cosmological observations?

If string theory is the correct theory which unifies gravity with the other forces of nature at a quantum level, it should determine the evolution of the earliest stages of the universe. I will discuss how stringy signatures of this early phase may be visible in current cosmological observations.

If large extra dimensions exist, microscopic black holes may be created in TeV particle colliders and in Earth's atmosphere by the collisions of ultrahigh-energy cosmic rays with atmospheric nuclei. The decay of these black holes could soon be observed at the Large Hadron Collider or the Pierre Auger Observatory. Monte Carlo codes have been developed to simulate these events. In this talk I will introduce two of these codes (CATFISH for the LHC and GROKE for the PAO), and discuss how mini black holes can be distinguished from standard model or susy events.

Danielsson, Ulf

New physics from inflation

I discuss how physics beyond the Planck scale and before inflation might leave an imprint on the primordial spectrum. There are interesting limitations connected with the information paradox that suggests unexpected new ways to test ideas on quantum gravity.

Ellis, John

Towards a Phenomenology of Quantum Gravity?

This talk will review proposed tests of ideas about quantum gravity, including searches for quantum decoherence, probes of the possible energy-dependence of the velocity of light, and the nature of vacuum energy. Motivations will be drawn from a non-critical string theory framework.

Giddings, Steven

Black hole production at high energies

I will survey some of the physics of TeV-scale black hole production, as well as outstanding issues. I will also discuss some of the conceptual issues surrounding high-energy black hole production.

Girelli, Florian

Symmetry deformation from quantum relational observables

Observables in (quantum) General Relativity can be constructed from (quantum) reference frame -- a physical observable is then a relation between a system of interest and the reference frame. A possible interpretation of DSR can be derived from the notion of deformed reference frame (cf Liberati-Sonego-Visser). We present a toy model and study an example of such quantum relational observables. We show how the intrinsic quantum nature of the reference frame naturally leads to a deformation of the symmetries, comforting DSR to be a good candidate to describe the QG semi-classical regime.

Hinterleitner, Franz

DSR and Classical Gravity

The talk gives a brief overview over different versions of doubly or deformed special relativity (DSR) and its motivation, which comes from the occurrence of a fundamental invariant length in quantum gravity (QG). Despite its QG origin, DSR is a modification of flat space geometry without explicit notion of gravity. In the literature there is a considerable amount of work done to probe deformations of special relativity in classical and quantum mechanics and quantum field theory without taking into account intermediate steps between QG and flat space, like general relativity or quantum field theory in curved space. The more special part of this contribution makes one step into this gap by comparing the DSR modifications of simple quantum scattering of a particle in flat space with the modifications caused by a weak classical gravitational field.

Holman, Richard

Imprinting Short-Distance Physics on Long Distance Observables: Can this be consistent?

We use the example of inflationary physics to discuss the possibility that short distance physics might be imprinted on long-distance observables. In particular, we focus on issues involving decoupling in field theory.

Husain, Viqar

Gravitational collapse in quantum gravity

I will describe work aimed at understanding the dynamics of gravitational collapse in a fully quantum setting. Its emphasis is on the role played by fundamental discreteness. The approach used suggests modifications of a black hole's mass loss rate and thermodynamical properties. Numerical simulations of collapse with quantum gravity corrections indicate that black holes form with a mass gap.

Laemmerzahl, Claus

Possible strategies for the search for quantum gravity induced effects

Accordingly, a first approach for the search of QG effects is to search for violations of the EEP. We will describe in a systematic approach given by the structure of the EEP the huge variety of experiments which are needed to test the EEP and report on the present status of these experiments. When available predictions from quantum gravity inspired scenarios will be presented, too. We also describe experiments in the quantum domain. After the main experimental part we also make short remarks on the strategies of a search for quantum gravity effects, on the magnitude of the expected effects, and on criteria of what can be regarded as a quantum gravity effect. Finally we give some information about which experimental accuracies can be expected in the future.

Landsberg, Greg

Out-of-this-World Physics: Probing Quantum Gravity in the Lab

In this talk I'll give an overview of various experimental ways of searching for large, small, and warped extra dimensions in space. I'll discuss astrophysical and cosmological constraints, as well as measurements of gravity at short distances. The most powerful way of searching for extra dimensions is offered by particle accelerators: the existing Tevatron collider and particularly the Large Hadron Collider scheduled to start running next year. To se the stage for the following talks, I'll give a brief introduction to mini-black hole production in high-energy collisions, including recent results on Randall-Sundrum black holes.

Liberati, Stephano

New constraints on Planck-scale Lorentz Violation in QED from the Crab Nebula

We set constraints on O(E/M) Lorentz Violation in QED in an effective field theory framework. A major consequence of such assumptions is the modification of the dispersion relations for electrons/positrons and photons, which in turn can affect the electromagnetic output of astrophysical objects. We compare the information provided by multiwavelength observations with a full and self-consistent computation of the broad-band spectrum of the Crab Nebula. We cast constraints of order 10-5 at 95% confidence level on the leptons Lorentz Violation parameters.

I will discuss possible tests of the grainularity of space including modified dispersion relations in the formation of white dwarfs and neutron stars and constraints on a stochastic direction field from atomic system tests.

Quantum Gravity may be entirely unconventional as a theory, leading to completely unfamiliar (compared to other fields of physics) and unexpected experimental signatures. One particularly interesting avenue for research in that field is the study of models in which quantum gravity operates as a decoherening ``foamy space-time medium'', with which ordinary propagating matter interacts. In such theories, which appear to involve the evolution of pure quantum mechanical states to mixed ones, at an effective low-energy level, the CPT operator of the effective low-energy field theory is ill defined, at least in its strong form, as argued in a theorem by R. Wald (1980) . This induces ``Microscopic Time Irreversibility'', a fundamental ``arrow of time'' in the effective theory. Experimentally of course, this arrow may not be observable: one may face a situation in which the experimentally accessible subspaces of quantum-mechanical states are decoherence-free subspaces, such that the relevant observables appear to be CPT symmetric, despite the strong form of CPT violation. This can happen, for instance, if cancellations of the ``anomalous'' CPT Violating terms between particle and antiparticle sectors occur. However, there are concrete quantum-gravity models of space time foam (some within the context of (non-critical) string theory), in which there are clear, and possibly unique (``smoking-gun'' type), experimental signatures of such an intrinsic CPT violation, manifesting themselves in induced modifications of the Einstein-Podolsky-Rosen (EPR) correlations of entangled states of neutral mesons in the appropriate meson factories. In the talk I will review the situation in some detail, discussing some indicative estimates of the effect, within some specific (non-critical) string models of space time foam for concreteness, as well as outlining the current experimental limits in phi- and B-meson factories and prospects for improvement in upcoming meson facilities, such as a possible upgrade of DaPhiNe. As I will argue, some models of this type of intrinsic CPT Violation may be falsified in such upgraded facilities.

Quantum fluctuations of spacetime give rise to quantum foam, and black hole physics dictates that the foam is of holographic type. One way to detect quantum foam is to exploit the fact that an electromagnetic wavefront will acquire uncertainties in direction as well as phase as it propagates through spacetime. These uncertainties can show up in interferometric observations of distant quasars as a decreased fringe visibility. The Very Large Telescope interferometer may be on the verge of probing spacetime fluctuations which, we argue, have repercussions for cosmology, requiring the existence of dark energy/matter, critical cosmic energy density, and accelerating cosmic expansion in the present era. We speculate that, in the framework of holographic quantum foam, the dark energy is composed of an enormous number of inert ``particles'' of extremely long wavelength. These ``particles" necessarily obey infinite statistics (quantum Boltzmann statistics) in which all representations of the particle permutation group can occur. For every boson or fermion in the present observable universe there could be ~ 1031 such ``particles".

Niemeyer, Jens

Inflation with a Cutoff: Proposals and Problems

The possible existence of a physical UV cutoff in dynamical spacetimes raises a number of conceptual and practical questions. If the validity of Lorentz Invariance is considered unreliable above the cutoff, the creation or destruction of quantum modes and the choice of their initial state need to be described explicitly. It has been proposed that these trans-Planckian effects might leave an oscillatory imprint on the power spectrum of inflationary perturbations. However, taking into account the fluctuations of the cutoff, the signal is smeared out beyond recognition.

Sudarsky, Daniel

The quantum origin of the cosmological structure: an arena for quantum gravity phenomenology

I will review the shortcomings of the standard account of the origin of anisotropies and in-homogeneities in inflationary cosmology. I will argue that something beyond the established paradigm of physics in needed for a satisfactory explanation of the process by which the seeds of structure emerge from the inflaton vacuum and will consider the application of a generalization of the ideas of R Penrose about a quantum gravity induced dynamical collapse of the quantum mechanical state of a system as a promising avenue to address the issue. I will show i) that the proposal offers paths to test the viability of rather specific ideas about the mechanism of collapse, ii) that generically it can led to some precise features in the primordial spectrum of density fluctuations, which can in turn be looked for, in the observational data, and used to set bounds on certain aspects the quantum gravity phenomenology, and iii) that it leads to other rather robust predictions that can be confronted with experiments.

Visser, Matt

Emergent dispersion relations --- lessons for quantum gravity.

The dispersion relations that naturally arise in the known emergent/analogue spacetimes typically violate analogue Lorentz invariance at high energy, but do not do so in completely arbitrary manner. This suggests that a search for arbitrary violations of Lorentz invariance is possibly overkill: There are a number of natural and physically well-motivated restrictions one can put on emergent/analogue dispersion relations, considerably reducing the plausible parameter space.

Weinfurter, Silke

Mass-generating mechanism for Nambu-Goldstone bosons in emergent spacetime and its application for quantum gravity phenomenology.

Effective field theories (EFTs) have been widely used as a framework in order to place constraints on the Planck suppressed Lorentz violations predicted by various models of quantum gravity. There are however technical problems in the EFT framework when it comes to ensuring that small Lorentz violations remain small -- this is the essence of the "naturalness" problem. Herein we present an "emergent" space-time model, based on the "analogue gravity'' programme, by investigating a specific condensed-matter system that is in principle capable of simulating the salient features of an EFT framework with Lorentz violations. Specifically, we consider the class of two-component BECs subject to laser-induced transitions between the components, and we show that this model is an example for Lorentz invariance violation due to ultraviolet physics. Furthermore our model explicitly avoids the "naturalness problem", and makes specific suggestions regarding how to construct a physically reasonable quantum gravity phenomenology.

Naturalness problems that could be signaling the necessity a completion of an effective field theory with the introduction of an otherwise overlooked ingredient. The cosmological constant problem can be seen as a signal that the EFT for gravity, general relativity, is not correctly including the gravitational properties of the vacuum.

Analogue models of gravity have been a test field for many phenomena in quantum field theory on curved spacetimes. It was recently recognized that in some situations they can also provide toy models of emergent gravity. We shall review here such a model and address within it the nature and scale of the cosmological constrant trying to draw general lessons for emergent gravity scenarios.

I discuss the status of Quantum Gravity Phenomenology, focusing separately on the 3 key areas: ability to discover, ability to constrain, and ability to falsify. And I stress the importance of adopting carefully taylored test theories as a remedy to difficulties encountered when comparing experimental evidence to theory evidence.

I discuss how physics beyond the Planck scale and before inflation might leave an imprint on the primordial spectrum. There are interesting limitations connected with the information paradox that suggests unexpected new ways to test ideas on quantum gravity.

We use the example of inflationary physics to discuss the possibility that short distance physics might be imprinted on long-distance observables. In particular, we focus on issues involving decoupling in field theory.

If string theory is the correct theory which unifies gravity with the other forces of nature at a quantum level, it should determine the evolution of the earliest stages of the universe. I will discuss how stringy signatures of this early phase may be visible in current cosmological observations.